The electrolytic production of chlorine, alkali metal hydroxide liquor and hydrogen from alkali metal chloride involves the use of naturally-occurring salts which necessitates removal of the accompanying impurities. For example, where rock salt is used in an electrolysis cell having a closed brine circuit, the polyvalent cations present in the salt have to be removed by alkaline or carbonate precipitation and the sulphate ions by precipitation with barium or calcium, for example. Detailed information on the electrolysis process and on brine purification may be found in the publication by Hund & Minz entitled "Chlor, Alkalien und anorganische Chlorverbindungen (Chlorine, Alkalis and Inorganic Chlorine Compounds)" in Winnacker, Kuchler, "Chemische Technologie", Vol. 2 "Anorganische Technologie I" pages 379-424, more particularly pages 421-424, and also in the secondary literature cited therein, or in J.S. Sconce: Chlorine. p. 135 ff (1962).
The present invention relates specifically to a process for removing sulphate ions from an alkali metal chloride brine in membrane electrolysis using ion exchangers.
Previously, even the most selective ion exchangers were unable to remove sulphate ions from a concentrated alkali metal chloride solution. The so-called "weak brine" effluent flowing from the electrolysis cell also contains a concentration of alkali metal chloride (approximately 200 g of alkali metal chloride per liter) which is too high to obtain a satisfactory ion exchanging effect. By contrast, highly concentrated alkali metal chloride solutions are suitable for removing sulphate ions from an ion exchanger charged with sulphate ions and thereby regenerating the ion exchanger.
In membrane electrolysis, the brine circuit is not completely closed with regard to water. As they pass through the membrane, sodium ions remove water from the anode compartment in the form of a hydrate shell. This hydrate water and the water discharged from the cell as water vapor with the gaseous chlorine needs to be returned to the circuit to maintain the necessary brine volume.